Environmental concerns have motivated the implementation of emission requirements for internal combustion engines and other combustion systems throughout much of the world. Catalytic converters have been used to eliminate many of the pollutants present in exhaust gas; however, a filter is often required to remove particulate matter, such as, for example, ash and soot. Wall-flow particulate filters, for example, are often used in engine after-treatment systems to remove particulates from the exhaust gas.
Such particulate filters may be made of a honeycomb-like substrate with parallel flow channels or cells separated by internal porous walls. Inlet and outlet ends of the flow channels may be selectively plugged, such as, for example, in a checkerboard pattern, so that exhaust gas, once inside the substrate, is forced to pass through the internal porous walls. The porous walls retain a portion of the particulates in the exhaust gas that passes therethrough. Particulate capture by the porous walls can occur in two different stages: at first, inside the porous wall (referred to as deep-bed filtration), and later, on the porous wall in the flow channels (so-referred to as cake-bed filtration). In this manner, wall-flow particulate filters have been found to be effective in removing particulates, such as, for example, ash and soot, from exhaust gas, providing relatively high filtration efficiencies throughout most of a filter's operation (e.g., providing close to 100% filtration efficiency upon onset of cake-bed filtration.) Particulate matter (PM) emission standards can, therefore, generally be met with relatively high levels of engine-out PM, which initiate an early onset of cake-bed filtration within the particulate filter.
Depending on engine calibration and the types of components used within an engine's after-treatment system, a particulate filter may, however, run in a wide range of engine-out NOx to engine-out PM (NOx/PM) ratios. A relatively low to medium NOx/PM ratio may, for example, result in the early onset of cake-bed filtration within the filter, whereas a relatively high NOx/PM ratio may result in a delayed onset of cake-bed filtration or even no cake-bed filtration within the filter. High NOx/PM ratios, for example, are generally coupled with high exhaust temperatures, which in turn tend to generate high passive regeneration rates (i.e., compared to soot accumulation rates) within the filter. Such conditions can lead to uneven soot distribution on the flow channel walls, thereby restricting the filter's operation to deep-bed filtration within part (or all) of the filter's volume. Thus, when a particulate filter is operating under high NOx/PM ratios, the filter's particle number (PN) based filtration efficiency may suffer, thereby increasing particle number slip from the filter (i.e., the number of particles that do not get trapped by the filter and are therefore emitted may increase due to the loss of cake-bed filtration within the filter).
To meet updated emission requirements, which may, for example, regulate both PM mass and PM number, it may therefore be desirable to provide a method of controlling the operation of a particulate filter to maintain particle number slip from the filter below a predetermined threshold.